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That is usually electrical. Depending in the vehicle, a spark plug wire, a spark plug, or a coil, if individual coil.
This means that spark plug number 3 is misfiring. You may just need a new plug. Replace #3 and see what happens. If your check engine light has been on, it should go off after a few miles with new plug.
It means that more than one cylinder has a misfire history. PO301 would be cylinder number 1 misfire. PO302 would be number 3 cylinder misfire.
It means # 3 cylinder has a misfire. My advice is to replace all the spark plugs, and check the spark plug wires carefully.
Number 3 cylinder is misfiring, it could be for several reasons such as a bad spark plug, a bad spark plug wire, a bad distributor cap, a bad rotor, a bad fuel injector, low compression due to a burned valve, large vacuum leak,
Were the spark plug wires replaced? Even a pinhole in a plug wire can leak enough spark to cause a misfire. Try swapping that wire with another similar length wire from another plug and see where the misfire is then. If the misfire moves to the new cylinder or ceases completely, you'll know the wire was bad.
a bad coil or plug to be safe change both
PO303 indicates a misfire on cylinder #3. Normally caused by bad spark plug or plug wire.
P0303 is cylinder #3 misfire. P0300 would be random misfire. Either way you need to diagnose the case of a misfire. Check spark plugs, plug wires(if equipped), cylinder compression, fuel inectors, and wiring.
Usually the occasional spark plug that needs to be cleaned and gapped.
Try replacing the plugs and wires
P0303, cylinder #3 misfire. Check plug, wire, coil, etc.